1. Field of the Invention
This invention relates to wafer handling apparatus for use in a semiconductor processing system and, more particularly, to a pick up wand for lifting wafers without contact with the top or bottom surfaces of the wafer.
2. Description of the Prior Art
Many different types of semiconductor processing systems require the use of wafer handling systems or wafer transport mechanisms. The more widely used processing systems will be briefly described below. Chemical Vapor Deposition (CVD) is the formation of a stable compound on a heated substrate, such as a wafer, by the thermal reaction or deposition of various gaseous compounds. Epitaxial deposition is the deposition of a single crystal layer on a substrate (often of the same composition as the deposited layer), whereby the layer is an extension of the crystal structure of the substrate. Another example of CVD is generally classified as metallization wherein the processed silicon substrates have the metal connectors and the like deposited thereon. In an ion implantation process, selected ions of a desired dopant are accelerated using an electrical field and then scanned across the surface of a wafer to obtain a uniform predeposition. Batch processing systems involve the deposition of more than one substrate or wafer at a time.
In the batch processing, the wafers are carried in boats and the boats are usually loaded and unloaded as by use of tweezers, hand-held vacuum pick ups and the like. Loading robots may be used to transfer multiple wafers simultaneously. While batch processing systems have been used extensively, the modern trend is toward the use of single wafer transport systems in order to process ever larger semiconductor wafers having diameters of over 30 centimeters. These larger wafers can contain many more circuits and much more complex circuits than were heretofore possible. While single wafer systems have less throughput than batch processing systems, attempts are being made to speed up the single wafer processes, to develope higher yields, to avoid problems such as particle contamination and to increase uniformity and quality.
Most known single wafer transport mechanisms can be adapted for use in various types of semiconductor processing systems. Such transport mechanisms include the following. In a gravity feed transport system, the wafers are stacked in a supply receptacle, which receptacle is supported in an elevator at an angle to vertical; the wafer within the receptacle is free to slide out and along an inclined ramp to a vacuum mandrel. A second inclined ramp is provided to permit a processed wafer to slide down the second ramp into a receiving receptacle. The disadvantages of this type of mechanism include the lack of positive feed; the material placed onto the wafer can come off on contact with the ramps; contaminating particles may be generated by the ramps; and, it may be limited to a single size wafer. Another type of semi-automatic mechanism for transporting wafers utilizes air bearings. The wafers are maintained horizontal and are transported to and from the processing area upon a cushion of air. This type of mechanism has proven to be highly unreliable and includes many moving parts subject to breakage and maintenance down time. Foreign material may enter and damage the air bearings or reduce their effectiveness. Other transport mechanisms utilize air cushion guidance devices where the problem of cleanliness of the air and of the turbulence produced by the air cushion are quite significant. The settling of airborne particles onto the top surface of the wafers is difficult to avoid in air cushion systems. Further, lateral guard rails must be used, and contact between the edges of the wafer and the guard rails occurs frequently and may result in unacceptable contamination or damage to the wafers. Finally, only one size of wafer can be handled without significant modification and down time occurs when the wafer receptacles are being replaced manually.
Various mechanical transport systems have been used. One system uses a rotating carousel in combination of supply and receiving slides. Another system uses a belt drive transport to discharge the wafers from a supply cassette and at various other transfer points. As the cassettes are discharged successively from the bottom and loaded in reverse order, impurities often drop from the bottom of one wafer onto the top of the wafer beneath it. Additional problems arise at the transfer points because the transport motion of the wafers is terminated by stoppers which can result in previously deposited layers being spalled off or chipped off to further contaminate the wafer surfaces. Other wafer handling systems utilize a spatula or shovel type pick up to slide under the wafers or move under the wafers and come up through the cassette track spaces to pick up the wafers and carry them to the next location.
An arm type system utilizes a vacuum chuck positionable under a wafer for attachment to the underside of the wafer by producing a vacuum at the point of contact. The wafer is lifted out of the cassette and carried to a processing station or the like; this system cannot place a wafer on a flat continuous surface. Damage often results from the mechanical contact between the vacuum chuck and the wafer.
In summary, the trend of the prior art is toward single wafer processing systems. A key factor in automating such systems lies in improving the wafer transport mechanism. Furthermore, the critical problems presented by particle contamination become ever more important as wafers become larger and larger and as circuits become more and more complex. None of the known systems are sufficiently clean to enable their use in a completely automated processing system, nor do they avoid touching the top and/or bottom surfaces of the wafer.